Deleting "Birthday" Calendar

I synchronize my iPhone with my Outlook calendar (and contacts) from my PC which runs Windows XP. When I upgraded to the iPhone 4 last week, a new calendar came up on my new phone called "Birthdays", and I have no idea where this came from. It currently has no events / entries. The "calendar" calendar is where all of my data in Outlook resides, but it is now displayed in red, or with a red dot (depending on the way I want to view it on the screen).

How do I delete the "birthdays" calendar?